1. Field of the Invention
This invention relates to the field of X-ray inspection systems and more particularly those inspection systems which use coherently scattered X-rays to detect the presence of explosives, incendiary materials, prohibited drugs or other illegally transported substances (hereinafter collectively referred to as "target material") in a container, the presence of imperfections or impurities in a material or object or the non-destructive, spatially-resolved, chemical analysis of an object.
2. Discussion of Prior Art
As the problems discussed in relation to known inspection systems below are equally valid for container, object and material inspection systems they will be discussed only in relation to container inspection systems. The term "container" shall hereinafter be taken to refer to both the container and its contents.
Many systems which utilise X-rays have been developed for the inspection of containers such as baggage and parcels. These include dual energy transmission imagers, backscatter imagers and computer-aided tomography (CAT) scanners all of which essentially yield effective atomic number and density information in two or three dimensions. However, these systems are inherently chemically non-specific and rely heavily on operator vigilance and interpretation during visual inspection of the spatial distribution of the contents of the containers and on other indicators such as the presence of initiating devices in the case of explosive target materials. A problem here is that many kinds of target material are not easily identifiable by their shape since they can be easily moulded or packed inside other items within the container to mask their presence. Furthermore, initiating devices are often masked by adjacent clutter within the container. These materials, and more specifically explosive devices, are therefore not readily detectable using these systems.
One known system which overcomes this problem is described in the U.S. Pat. No. 4,751,722 (EP 0,153,786). This discloses a system which analyses the intensities of X-rays which are coherently scattered through different small angles when passing through a container. This information is then used to generate an angle dispersive X-ray spectrum which is characteristic of the ordered molecular structure of the material responsible for the scattering. The X-ray spectrum may then be passed to a data processing system where an automatic comparison is made with stored spectra characteristic of the target materials of interest in order to determine whether or not these target materials are present within the container.
However a problem with this system is that the contents of a container can only be resolved in the two dimensions of the scattering voxel (volume element) in the plane perpendicular to the direction of travel of the unscattered X-rays, ignoring the third dimension in the direction of the unscattered beam, through the depth of the container. Where either the container or the target material has a third dimension greater than a few centimeters then depth resolution is required in order to satisfactorily identify the presence of a target material. This is because coherent scattering over a range of scattering angles from non-target materials at different depths within a container cannot be differentiated from the scattering caused by localised target material at any given angle of interest thereby causing the generated X-ray spectrum to become confused and identification of target material difficult.
A further problem with this earlier system is that the low X-ray flux generated by presently available monochromatic sources results in too great an inspection time, making it impractical to use in situations where a large number of objects of large size need to be rapidly inspected eg. such as for containers in airports. However, the use of a high intensity polychromatic source would degrade the material discrimination capability of the system.
UK patent No. 1,463,054 provides a means of examining a body, eg. a human skull, in which the body is mapped in two dimensions including the vertical dimension, for the purpose of determining the position of scatter centres within the body. Whilst this equipment is capable of scanning through the depth of the body being examined its use of conical collimators or annular detectors limits the flexibility of the system in a manner which is significantly disadvantageous for the purposes of materials detection as it is not possible to arrange for simultaneous inspection of more than one closely spaced voxel using a single X-ray source. The method is inherently incapable of providing the chemically specific information which is required of a detection system of the type with which the present invention is concerned.
Another known X-ray detection system is shown in FIGS. 1 to 4 of U.S. Pat. No. 4,956,856. This uses long thin X-ray detectors, which must be longer than the body to be examined, to measure scattering spectra from small volumes within an object in three dimensions. This patents concedes that for the inspection of baggage such long detectors would have insufficient spatial resolution for the accurate determination of scattering spectra. This would lead to poor material discrimination. Similar results would be obtained using these detectors for the inspection of any object of moderate diameter, even for those objects with diameters significantly less than typical luggage. Furthermore, such an equipment could not operate with good material discrimination using a polychromatic X-ray source, such as an X-ray tube, as the energy resolution of the scintillator based X-ray detectors which are used is insufficient. A monochromatic source would be required for good discrimination with similar implications for inspection times as noted for U.S. Pat. 4,751,722 above.
The embodiment depicted in FIGS. 5 to 7 of U.S. Pat. 4,956,856 uses a gamma (or Anger) camera based X-ray detector which cannot be implemented with sufficient photon energy resolution for good material discrimination as this detector is similarly scintillator based. In this invention it is also necessary to displace the object under inspection (or alternatively the source, collimator system and detector) along two perpendicular axes, or the object along one axis and the source, collimation system and detector along another perpendicular axis, in order to carry out full three dimensional inspection of the object. This is mechanically complex and time consuming. A further difficulty is that the number of regions of the path of the unscattered X-ray beam through the object which can be independently inspected will be limited by the diameter and spatial resolution of the gamma camera.
U.S. Pat. No. 5,007,072 (EP 0,354,045) describes another X-ray inspection system, again inspecting an object in only two dimensions with the drawbacks outlined above for U.S. Pat. No. 4,751,722 and further detecting only at a set angle and measuring the wavelength spectrum of radiation scattered at that angle. To do this also requires the use of expensive and complex solid state germanium or silicon detector arrays operating at cryogenic temperatures. Likewise the detector disclosed in U.S. Pat. No. 5,265,144 is based on measurement of the energy of scattered radiation at a fixed scattering angle. Moreover in the case of this detector it is focussed on a small area within the object being examined and is only able to inspect voxels arranged in one dimension through the object by translation of the relative positions of the detector and collimator systems and the object. Full three-dimensional inspection with this system would require translation of the object or of the source, detector and collimator systems in a total of two further perpendicular dimensions. This would not provide a practicable equipment in the context of a baggage inspection system.
Finally, a further patent, U.S. Pat. No. 4,754,469, discloses a detection equipment which employs a monochromatic X-ray source, with consequent long inspection periods, or expensive and complex solid state cryogenically cooled germanium or silicon detector semiconductor arrays for good material discrimination. Translation of the object under inspection or of the X-ray source, collimation system and detector arrays together in a combined total of two mutually perpendicular axes is also required for two dimensional analysis. Three dimensional inspection would only be achievable by additionally displacing the relative positions of the object or of the truncated detector collimator.